This invention relates to the provision of improved architectural sheet products, particularly products which are anodized and to methods of producing such.
Architectural aluminum sheet products have enjoyed substantial commercial success and are readily observed covering many large high-rise buildings. One popular method of treatment applied to these products is anodizing in an acidic electrolyte to provide a durable, decorative anodized coating. Aluminum alloy products in alloys 1100 and 5005 have seen wide use in this application because of modest cost, good formability, adequate strength, good rolling and mill finishing properties and desirable response to anodizing treatments.
However, one problem in these alloys and similar alloys involve the formation of bands of inconsistent color or texture appearing along the length of the anodized sheet usually at certain regions across its width. This condition, a rejectable defect, is sometimes referred to as a "tiger stripe" defect and speaking more technically, is often referred to as structural streaking and is apparently caused by inconsistent or non-homogeneous distribution of ingredients or phases across the width of the sheet. Homogenizing treatments and other treatments performed after casting are not effective to cure this problem, although some improvements have been achieved through better control of the casting operation such as better grain refinement and the use of low molten metal levels in continuous casting molds, often referred to as low head casting, which achieves very high ingot surface region chill rates. Still another casting improvement lies in electromagnetic casting. However, there remains substantial room for improvement in this important application in that rejections for structural streaking defects still persist.
The objectionable condition is aesthetic rather than genuinely structural, and like other aesthetic defects, the condition is objectionable simply because it looks objectionable. If sheet products are rolled thin enough, for instance less than 0.015 or 0.02 inch thick, to elongate the condition over more surface, the degree of streaking defects appears to lessen. However, for relatively thick sheet of over 0.03 or 0.04 inch, the structural streaking problem persists and it is thick sheet of about 0.04 to 0.15 or 0.25 inch which is usually needed for architectural applications.